Recently, with the spread of compact cameras, autofocus single-lens reflex cameras and film-incorporated cameras, there has been a strong demand for a silver halide color photographic material with high sensitivity and superior image quality. Consequently, need for improvement of photographic performance of silver halide emulsions has also become severe, and further higher level requirements for photographic performance including high speed, superior graininess and sharpness have also been made.
In response to these demands, U.S. Pat. Nos. 4,434,226; 4,439,520; 4,414,310; 4,433,048; 4,414,306 and 4,459,353 disclose techniques of using tabular silver halide grains, which is known to bring about advantages such as enhancement of sensitivity including enhanced spectral sensitization efficiency with a sensitizing dye, improved sensitivity/graininess, improved sharpness and covering power due to specific optical property of the tabular silver halide grains.
JP-A 7-191425 (herein, the term "JP-A" is referred to as unexamined, published Japanese Patent Application) describes tabular silver halide grains with an aspect ratio of less than 5 and being internally reduction-sensitized, in which a variation coefficient of twin plane spacing (x) and a variation coefficient of grain thickness(y) meet the relationship, 0.7.ltoreq.y/x.ltoreq.2.0. These tabular grains, however, were found to provide insufficient response to recent high level requirements, and still further enhanced photographic performance is desired.
Relating to this trend of higher speed and higher image quality, the demand for improvement in pressure characteristics of silver halide light sensitive photographic materials continuously increases. There have been attempts for improving pressure characteristics by various means, in which techniques of enhancing resistance to stress of silver halide grains is generally thought to be preferable and valid in practice, rather than techniques of incorporating additives such as a plasticizer. In response to this demand, there have been extensively studied photographic emulsions comprised of core/shell type silver halide grains having a high iodide silver iodobromide stratum. In particular, there is paid attention on a silver iodobromide emulsion comprised of core/shell type grains internally having a high iodide phase containing 10 mol % or more iodide.
Techniques of improving pressure characteristics of core/shell type grains are disclosed in JP-A 59-99433, 60-35726, and 60-147727. JP-A 63-220238 and 1-201649 also disclose techniques of improving graininess, pressure characteristics and exposure intensity dependence as well as sensitivity by introducing dislocation lines within the grain. Further, JP-A 6-235988 discloses multilayered structure type, monodisperse tabular silver halide grains having a high iodide intermediate layer. These techniques, however, are still insufficient to meet recent high level requirements as a silver halide emulsion with high sensitivity, superior graininess and improved pressure characteristics.
As a technique of controlling the charge carrier within the silver halide grain, such as a free electron and positive hole, is known a metal-doping technique. For example, Leubner reported that doping of an iridium complex into the silver halide exhibited an electron-trap property The Journal of Photographic Science Vol.31, 93 (1983)!. JP-A 3-15040 discloses an iridium ion-containing silver halide emulsion, in which iridium ions are not present on the surface of silver halide grains and also a preparation method thereof. JP-A 6-175251 discloses a technique of improving both sensitivity and reciprocity law failure characteristics at 1/100 sec. exposure with in-plane epitaxial grains, in which an iridium compound is incorporated during the course of preparing the grains. JP-A 8-160559 discloses a technique of improving high intensity reciprocity failure characteristics with tabular silver halide grains in which less than 1/20 of the total content of a polyvalent metal compound (mol/mol of AgX) is contained in the outermost surface layer. However, any of these techniques is still insufficient in providing a photographic material with the sensitivity, image quality and high intensity reciprocity failure characteristics required in the market.